Fuel cells generating electric power by an electrochemical reaction of hydrogen with the air have recently drawn attention as the novel energy source. Fuel cells having solid electrolyte membranes include low-temperature-operation-type fuel cells, such as polymer electrolyte fuel cells, and high-temperature-operation-type fuel cells, such as solid oxide fuel cells.
The polymer electrolyte fuel cell has a solid polymer film, for example, a Nafion (registered trademark) film applied to an electrolyte membrane interposed between electrodes. A decrease in water content of this electrolyte membrane lowers the ion conductivity and increases the film resistance. Control of the film resistance in a practical range requires operation of these fuel cells in a relatively low temperature range that prevents extreme evaporation of the water content. The polymer electrolyte fuel cells are generally operated in a low temperature range of not higher than 150° C.
The solid oxide fuel cell has a zirconia thin film or another inorganic thin film applied to the electrolyte membrane interposed between the electrodes. The film resistance of this electrolyte membrane increases with a temperature decrease. Control of the film resistance in a practical range requires operation of these fuel cells in a relatively high temperature range. Reduction of the thickness of the electrolyte membrane naturally decreases the film resistance, but it is extremely difficult to form a dense thin film on the electrode of a porous material. Sufficient reduction of the membrane thickness is thus not attainable. The solid oxide fuel cells are generally operated in a high temperature range of not lower than approximately 700° C.
The fuel cells of these two different operation types are both cooled down by circulation of cooling water to be operated in the respective adequate temperature ranges as disclosed in Japanese Patent Laid-Open Gazette No. 2002-249303.
The cooling water circulated through the fuel cells is gradually heated up to have a higher temperature and a lower cooling efficiency in the downstream. The variation of the cooling efficiency inside the fuel cells causes a temperature gradient in the fuel cells and may lower the power generation efficiency. This problem is not characteristic of the fuel cells having the solid electrolyte membranes but is commonly found in various types of fuel cells.